Automatic control for photographic printing exposures



A. G. DENIS Sept. 11, 1934.

AUTOMATIC CONTROL FOR PHOTOGRAPHIC PRINTING EXPOSURES 2 Sheets-Sheet 1 Filed April 23, 1931 Zjwmntoz,

' Amandamnw,

'A.-G. DENIS Sept. 11, 1934.

AUTOMATIC CONTROL FOR -PHOTOGRAPHIC PRINTING EXPOSURES Filed April 23, 1931 2 Sheets-Sheet 2 Patented Sept. 11, 1934 AUTOMATIC CONTROL FOB PHOTDGMC PRINTING EXPOSURE5 Armand G. Denis, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y, a corporation of New York Application Amines, iosi, Serial No. starts 9 our. (on. semis) This invention relates to photography, and more particularly to photographic printing processes and apparatus of the automatic or semi-automatic types.

It has long been Known that, when printing a series of negatives of different values of average transmission, as good or better average results are obtained if the intensity of the source of light used for printing is so adjusted that the product of this (actinic) intensity and the average transmission remains constant, as when the exposures are determined icy the judgment of experienced operators.

Attempts have been made towards developing processes of printing in which the intensity of the source or" light would be automatically adjusted so as to satisfy this relation, empirically found to be desirable.

In the past, these attempts have in the main been directed to the use of a sensitive element for measuring the average transmission of the negative and to the use of methods in which each of the widely difierent readings of this element, corresponding to different values of the average transmission of the negative, would bring about an adjustment of the light source intensity to a predetermined value calculated or empirically found to result in exact compensation for the given transmission.

In practice these methods have been iar from satisfactory for the following reasons:

The measuring elements fail to follow rigidly defined response curves throughout wide ranges and differ individually from each other.

The lamps used as sources of light gradually vary in efiiciency and differ from each other.

The characteristic response curves of a light sensitive element and a light source can be matched to each other, throughout the wide range.

of variations of both which is necessary, only approximately and only by complicated circuits or methods.

The accuracy of control is affected and seriously disturbed by all accidental variations (such as line voltage variation) and losses of adjustment or efliciency in the circuits or mechanical systems used to connect the measuring element and the source of light; in other words these methods are prone to loss of adjustment and are not inherently self correcting or flexible.

It is an object of this invention to provide apparatus for the automatic regulation of the light source intensity in photographic printing, which is substantially free of the above faults and limitations; and to provide apparatus which will inappended claims.

stantaneously and automatically regulate the ra-= diant energy level incident on the negative in such a manner that the radiant energy transmitted by the negative is maintained at a substantially constant level.

It is an object of this invention, in a photographic printer, to apply a radiation-=sensitive measuring element to measure directly the radiant energy actually transmitted by the negative and incident on the positive, and to uti ize this measurement to maintain substantially constant the radiant energy incident on the positive. this respect this invention diiiers greatly from previous attempts in which the measuring ele ment was applied to measure the average transmission of the negative; leading to a wide variation in thereadings oi the measuring instrument and requiring the measuring element be operated through large portion of acteristic curve.

In this invention the measuring element is ap plied to measure a quantity which is, by 'c 1 ation of the invention itseli, maintainec'l subs n-= tially constant; the measming element therefore operates substantially constant re' d nsi and the shape or" its response curve-which no needs to be matched to the eharacte oi the source or light-hecomes no it is an object of this invention to ods and apparatus whereby the ele ment, while operating at substantially constant reading and allowing only negligible variations around a predetermined level of the radiant en ergy transmitted by the negative and incident on the positive, is able to control accurately and with negligible time lag large and sufdcient changes in the intensity of the light source to compensate for widely different negative transmissions.

Another object or" this invention is to provide a method and means for automatically controllhsg the printing of continuous film in such a manner that each successive frame oi the positive copy shall receive a substantially identical amount of illumination irrespectiveoi the density of the corresponding negative frame. Another object of this invention is to provide a method and means for automatically controlling the photographic printing operation in such a manner that the total light transmitted by the negative is kept substantially constant.

Other objects and advantages of this invention will be apparent from the following description when read in connection with the accompanying drawings, and its scope will lie-pointed out in the Referring to the drawings, fig. 1 shows in diagrammatic form an automatic printer circuit capable of carrying out this invention;

Fig. 2 illustrates the invention applied to a printer in which tungsten lamps are modulated for regulating the printing illumination;

Fig. 3 illustrates a simple manner in which the wattage of the tungsten lamps is indirectly conthe control element.

In the printer circuit schematically shown in Fig. 1, a suitable glow discharge tube 10 is adapted to furnish printing illumination to a printing plane. Positioned behind the printing plane is a photo-electric cell 11 of any conventional type polarized by the batteries 12 and 13. In series with the cell 11 is a coupling resistance 14 which may conveniently be of the order of 20 megohms. The anode of the cell 11 is connected to the grid of the tube 15 and both the anode and the grid are connected with the resistance 14 and the battery 3.2 to ground. The filament of the tube 3.5 connected directly to the ground so that the tube 15 is impressed with any variations in tentiei drop across the resistance 14:. The tube is is the input of an amplifying circuit of conventicna'i design, the output of which is connected to the glow discharge tube 10. The filament heat-er l. idings and the power supply connections 1" cneen omitted as they are well under oithe tube 15 at the input a low thermal inertia and is fed ts,

being present in appearing, greatly an; or" the amplifier, which 95 5 barge tube ill used as the source Thus it will be seen that the grid of 15 may be made to assume any potential, limits, relatively to the potential of moment. El, will further be clear to those familiar with amplir' mg circuits that there exists a well defined intensity of illumination falling upon. the photocell 11 which will result in a normal bias of the grid. of tube 15 and thus in operation of the amplifier at full amplification; that a sfnall increase in the intensity of illumination would be suficient to make the potential of the grid markedly negative to that of the filament and thus to cause the output of the amplifier to fall to a very low value; and that there exists a narrow range of intensities of illumination which may be termed the critical,range, within which-increases or decreases of the illumination small enough to be negligible in comparison with its absolute value will cause very large decreases or increases of the output of the amplifier.

In an actual printer, the photo-cell 11 and the glow discharge tube 10 would be brought close together while the path of light between them would be restricted to the aperture of a printing gate; the arrangement being such that the light from the glow tube would be distributed evenly over this aperture, and collected on the surface of the photo-cell after passing through the film under process of printing at the. gate.

The range of possible light intensities and the distance of the glow lamp from the gate would be so chosen as to suflice for correct printing, at the required speed, of negatives of the density range which it is desired to cover; while the sensitivity of thephoto-cell 11 and its distancefrom the gate would be so adjusted that the amount of illumination reaching the printing surface will, when it is adequate for correct printing, also bring the grid of the tube 15 within the critical region.

It will be obvious that if these conditions are atives placed in the gate, the intensity of the glow lamp must instantaneously become adjusted so that the illumination falling upon the photo-cell 13. is that required to bring the grid of the tube 15 within the narrowly defined critical range, and which by adjustment, has been made equal to that adequate for correct printing; and the intensity of the glow lamp must remain at all times correctly adjusted, irrespective of the density oi the negatives placed in the gate. For the illumination falling upon'the photo-oell ll tended to decrease appreciably, the potential of the grid of the tube 15 would approach that of the filament, thus causing the glow lamp to approach its maximum illumination which, by definition, is more than sufflcient -with the most dense negative to bring the grid of the tube 15 within the critical range.

nile the illumination falling upon the pin... ll ll. to'increase appreciably, the potential of of the tube 15 would tend to become markedly negative to that oi'the filanient, thus causing the glow lamp to approach its of illumination which, by definition, more than sufioient, with the least dense negative, to the grid of the tube 15 within the critical range.

In the control circuit above descr'bed it will be seen that the radiation sensitive element has been so placed and connected as to measure (iirectly the radiant energy actually transmitted by the negative rather than the average transmission of the negative, although it does take into account such average transmission, along with several other factors such as the intensity of the printing source. The sensitive element 11 measures, in fact, the result oi regulation, that is, the actual amount of printing illuminaticn reaching the positive or a quantity rigidly proportional to it.

Then, by definition, if this actual amount is to be kept substantially constant, it will be seen that, in the operation of the device, the sensitive element 11 will be operated at substantially constant reading due to the fact that very small increases or decreases in the light incident on this sensitive element around a predetermined value results in very large changes in the intensity of the source of illumination 10.

In using the control circuit just described it is sufiicient in order to obtain complete control to establish empirically. the measuring level at which the amount of illumination reaching the positive film results in correct exposure and then ,fulfilled, irrespective of the density of the negto connect the sensitive element with the variable source of light in such a manner and through such amplification that variations of the intensity of the source of illumination throughout its complete range and in the correct direction should result from variations of the reading of the sensitive element around its correct level sufllciently small to be negligible. From this it will be seen that the characteristic response curves of the sensitive element or of the source of illumination are of no importance and may be of any form. The characteristics of the amplifier connected to the tube, are also unimportant and any reasonable variation of any of the elements involved is automatically corrected.

In applying the above described control circuit to a practical printer, it is necessary only to insure that each frame shall receive exposure for a definite time, either by providing a uniform rate of motion for the film or by some suitable shutter arrangement. One arrangement which may be used for advancing the film will be described in connection with Fig. 5. Given a definite time of exposure for each frame, it will be obvious that the above described circuit will instantaneously and automatically regulate the intensity of the printing illumination in such a manner that each successive frame of the positive copy in course of printing shall receive a substantially identical amount of illumination, irrespective of the density of the corresponding negative frame, the intensity of the printing source being varied frame by frame so as to compensate accurately for the variations in the averagedensity of successive negative frames.

The circuit shown in Fig. 2 utilizes the same general method which has been described in connection with Fig. 1 and illustrates a form of automatic printer in which an ordinary tungsten lamp is used as the source of illumination and in which the wattage of the lamp is varied, in order to vary its illuminating intensity, through an electrical circuit free from moving parts and from appreciable time leg.

The use or a tungsten lamp as the source of illumination has several distinct advantages including easy replacement, smaller wattage for a given amount of illumination, wider range of variable actinic intensity without this range being limited by extinction or other discontinuous phenomena, and susceptibility to control by electrical methods involving no moving parts and free from appreciable time lag.

Although the tungsten lamp is a desirable and emcient source of light, there are two serious difilculties in using this type of lamp as a variable source of illumination for printing purposes, the

first of these being that the wattages required to.

operate a tungsten lamp capable of giving adequate printing exposure at normal printer speed to the densest negatives which are liable to be encountered in practice are relatively large, and an amplifier capable of handling these wattages directly would be impractical both from the standpoint or" size and cost. The second of these difficulties arises as the result of the large variation in spectral distribution-of the tungsten light as the wattage and consequently the color ternperature of the lamp are varied. It will be explained in the course of reference to Figs. 2 and 3, how these difiiculties have been avoided.

Referring to Fig. 2, the photo-cell ll, sensitive to medium or long wave radiation, is positioned so as to efiectively collect radiation from the source of illumination 10 transmitted through the negative and positive films moved through the printing gate 16. The photo-cell 11 is polarized by the battery 12 placed in series with a coupling resistance 14 and so connected to the input circuit of the tube 1'7 that variations in the potential drop across the resistance 14, small enough to be negligible in comparison with the total drop across 12, are suflicient to vary the potential 01' the tube 17 through its critical range.

The tube 17 is preferably one known commercially as a 227 and is here used purely as a direct current amplifier. The plate of the tube 17 is connected to the cathode through the plate resistor 18 and the battery 19 so that the small variations of potential drop across the coupling resistance 14 are caused to result in large variations (of opposite kinds) of plate current in the tube 17, which variations are used to control a full wave rectifying clrcuitcomprislng two tubes 20 and 21 respectively.

The filaments of the tubes 20 and 21 are energized by a common alternating current transformer winding (not shown) and the plates are connected to the terminals of a center tap high voltage winding of a transformer 22. The lilaments of the tubes and the center tap of the transformer 22 are connected through one winding oi each of the chokes 23 and 24., and are intended to produce direct current saturation of the cores of said chokes. The filaments are connected to the plate of the tube 17 and the grids are connected together and given, by connection to a suitable tap on the battery 19, a permanent bias such that, when the photo-cell 11 is in darkness, the potential of the grid and filament of the tubes 21 and '20 substantially coincide and the rectified output to the chokes 23 and 24 is a maximum. The variable output of the tubes 20 and 21 is applied to cause a variable degree of saturation of the iron cores of the two chokes 23 and 2a which automatically varies the wattage supplied to the source of illumination 10 as here inaiter described.

The simple manner by which this variable D. C.

output indirectly controls the variable wattage supplied to the tungsten light source 10 will be readily understood by reference to Fig. 3. The lamp 10 in series with an impedance consisting of the resistance 25 and the iron core inductance 23 in parallel is energized from a suitable constant voltage A. C. supply. The secondary winding of the choke 23 is connected to the variable D. (3. output of the rectifying circuit, and the satura tion of the iron core of the choke 23 is thus varied in accordance with the direct current flowing through the winding. Variation in the saturation of the iron core carries the inductance of the winding 23 through wide limits, and thus the current to the lamp 10 is varied through wide limits by relatively small current variations in the D. C. coil of the choke 23.

The percentage variations of inductance of the winding 23' possible by gradual saturation of the core are usually considerably more than sufficient to cause modulation of the lamp wattage through the range of illumination required in the printer. The resistance 25 usually permits of increasing appreciably the total amount of current handled by the circuit, and thus of using a lamp capable may also be used to permit of greater efliciency.

The arrangement shown in Fig. 3 has been described merely to explain the general method used for indirectly controlling a tungsten light source.

It is not, in this simple form, suitable for use in an automatic printer, because of the variation in the spectral distribution which accompanies changes in the color temperature of the tungsten lamp. In a practical printer, it is preferable to use a composite lighting unit which may well be made of two lamps as shown in Fig. 2, and in which the two lamps are simultaneously controlled in such a way that the total actinic radia-.

tion of the unit, and the radiation to which the photo-cell 11 is sensitive, remain substantially proportional throughout their range of simultaneous variation.

If two lamps are so connected that their currents, throughout their range of variation, differ by a substantially constant amount and if the Eight from these lamps is filtered so as substantialiy to exclude from the output of the printing lamp the radiations which might afiect the photo-cell or other measuring element, and from the output of the control lamp radiations which would affect the sensitive layer being printed, these two lamps may be combined in a unit, the combined output of which contains printing light and measuring light in unvarying proportions, and is therefore equivalent to a. source of radiant energy of constant spectral distribution. I

In Fig. 4 is illustrated a circuit arrangement by which the required relation may be obtained between the currents of the two lamps used in the lighting unit 10. The radiation from the lamp 27 in unit 10 is filtered by a red or yellow filter to remove all actinic radiations whilepassing the measuring light; and the radiation from the lamp 28 is filtered by a blue or other suitable filter to free it from such wave lengths as would affect the photo-cell 11, while passing radiation suitable for printing purposes.

Both ofthe lamps 2'7 and'28 are placed in series in a circuit identical to the one described in connection with Fig. 3 and in addition the lamp 2'7 and the resistance 25 are shunted by a fixed resistance 26; the choke 23, the inductance 23 and. the resistance 25 performing the same function as previously described.

The current passed by the lamp 28 divides itself between the resistance 26 and the lamp 27 and its associated variable impedance circuit. The lamp 28 is preferably of a voltage low in comparison with the voltage drop across the resistance 26 so that the 'voltage variations across the lamp 28 due to adjustment required for printing are negligible in comparison with the voltage drop across the resistance 26; the current, through the resistance 26 thus remains substantially constant and independent of the variations of the system.

It will be seen that the current. through the lamp 28 exceeds by a substantially constant value the current through the lamp 27. Obviously the constants of the circuit can be so chosen or adjusted that the required proportionality may be obtained between the filtered measuring" radiation of the lamp 27 and the filtered actinic radiation of the lamp 28.

The arrangement shown in Fig. 2 differs from the one just described merely in that the lamp 28 is shunted by the resistance 29 in order to obtain the required constants of operation. The

complete arrangement shown in Fig. 2 includes a printing gate 16 through which negative and positive film 'are adapted to be intermittently moved and a shutter 16' adapted to exclude the printing *light from the gate during said movement. Means for moving the film and operating the shutter are well known and, therefore, are not illustrated. When any one frame of the film is stationary in the gate 16 and the shutter 16' is open, radiation from the source 10 containing actinic and "measuring radiation in unvarying proportion passes through the film and is tive film during the fixed time of the exposure.

It is to be understood that the described arrangement represents only one method by which the required constant relation between the simultaneously variable Wattages of two or more lamps may be maintained. Any circuit including a plurality of lamps so connected and filtered that their respective wattages or currents are varied simultaneously and remain, throughout the range required for printing, proportional to quantities difiering by a fixed amount, and that their combined output, including actinic radiation and radiation suitable for measurement, remains in unvarying proportion, independent of the variations of the system as a whole, is intended to fall within the scope of this invention.

In Fig. 6 the invention is shown applied to a printer or enlarger for stills comprising a source of light 10 adapted to furnish printing light through a collecting lens 30, a negative 31, a projecting lens 32 and on to the positive 33 being printed. Suitably positioned between the negative 31 and the positive 33 is a partially reflecting and partially transmitting sheet 34, which may be of celluloid, or the like, for reflecting a, portion of the light transmitted by the negative through a collecting lens 35 on to the photo-electric cell 11. v

The partially reflecting surface of the member 34 reflects a known proportion of the illumination falling on the positive member 33 and this known proportion of the illumination is collected on the photo-electric cell 11 which controls the light source through a suitable amplifying circuit as described in connection with either Fig. 1 or Fig. 2. The response of the photo-electric cell 11 is utilized to vary the printing source of light 10 in such a way as to insure a definite level of radiant energy always to be falling on the sensitive layer of the positive 33. Thus it will be seen that for any given time interval a definite amount of illumination will have reached the positive 33. and exposures can be accurately determined in advance.

In the arrangements so far described the radiations activating the photo-electric, cell have been shown as emanating from the source utilized for furnishing the printing light. In Fig. 7 I-have shown a modification which may be used for printing either stills or motion picture film by the projection method in which a separate source of light is utilized in energizing the photo-electric cell in'accordance with the transmission of the negative being printed. As shown in Fig. 7 a condensing lens 41 collects light from a source of radiation 10 and transmits this light through a negative 42 to the positive 43 being printed, through the medium of a suitable lens 44. As shown, a second glow discharge tube 40 is connected in series with the main discharge tube so as to vary in accordance with and simultaneously with the glow tube 10. The tube 40 is positioned to transmit light through the condenser 41; the negative 42, and on to the photo-electric cell 11 at an angle sufilcient to prevent any of its rays reach ing the positive film 43. In this arrangement no adjustment of the circuit needs to be made to compensate for varying thickness and opacity of the positive film being printed.

As heretofore described, the photo-electric cell 11 operates through a suitable amplifying circuit to control the variations of the source of printing light 10. It is obvious that the source 10 may comprise the tungsten source as described in connection with Fig. 2.

In Fig. 5 is illustrated one arrangement for advancing positive and negative film through a printing gate 16 suitable for use in connection with the circuit described in connection with Fig. 1. Although any standard arrangement may be used, the one illustrated comprises feed sprockets driven by any suitable source of power, not shown, which are adapted to advance the negative film from the reel 51 and the positive film from the reel 52 at a uniform rate through the printing gate 16, after which the negative film is taken up on a suitable reel 53 and the exposed positive film is carried through any suitable developing tank 54.

As described in connection with Fig. l, the glow discharge tube 10 and the radiation sensitive element 11 are so connected that the glow lamp 10 is so modulated that the radiant energy reaching the positive film is always maintained at a constant level. With the negative and positive moving at a constant rate of speed, it will be seen that a uniform exposure is given for the entire positive film, regardless of any variations in density which may occur in the negative film.

While I have described in detail several ways of practicing my invention to comply with the requirements of the statutes I wish it understood that I do not intend to be limited to the exact methods or apparatus described inasmuch as in view of the disclosure obvious modifications will readily suggest themselves to those familiar with this art without departing from the spirit of my invention er the scope of the claims herein.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Photographic printing apparatus including a source of light, means for advancing afilm of variable translucenoy between said light and a light sensitive element, means governed by the consequent activation of said element for varying the intensity of said light source and means associated with said source for causing said variation to occur without disturbing the spectral distribution of the light.

2. In photographic printing apparatus, anv ele ment sensitive to non-actinic light, means for directing a photographic record and a layer sensitive to actinic light in front of said element, a composite source of light for directing light through the record and layer and upon said element including two tungsten lamps, a porn actinic light transmittingfilter associated with one of said lamps, an actinic light transmitting filter associated with the other of said lamps and means controlled by the response of said senlight of constant intensity falling on a sensitizedlayer being printed which comprises passing a composite beam of radiant energy through a photographic record and:the sensitized layer, causing the non-actinic component of the beam to produce a current varying with the translucency of the record, and utilizing said varying current to modulate said composite beam without disturbing its spectral'distribution.

4. In the art of photographic printing the method of controlling the actinic radiation falling on a sensitized layer being printed which comprises passing non-actinic radiation through the photographic record being printed, modulating said non-actinic radiation in accordance with the translucency of the record and simultaneously modulating the actinic radiation to maintain it in unvarying proportion with the nonactinic radiation.

5. Photographic printing apparatus including means for holding superposed negative and positive films in an exposure window, a composite source of light in front of the Window including a source filtered to transmit actinic light and a source filtered to transmit non-actinic light, a light sensitive element behind said window adapted to be activated in accordance with the non-actinic light impinging thereon and means controlled by said element for varying simultaneously the two components of said composite source of light.

6. Photographic apparatus or" the variable intensity constant time type comprising a source of tensity of the light transmitted by said record 7 at said window and means controlled by the output circuit for varying the intensity or" the light falling on the printing window, the complete arrangement forming a photo-electric governor operative to maintain substantially constant at a redetermined value the potential on said control electrode, whereby the printing light falling on said layer is of a predetermined intensity.

'7. Photographic printing apparatus for controlling the intensity of an exposure given a sensitized layer through a photographic record of varying translucency, comprising a source of light for furnishing the exposing light, means for varying the intensity of the light falling on the record, amplifying means for controlling said light varying means and a light sensitive element for regulating the input to said amplifying means in accordance with the intensity of the exposing light transmitted by said record whereby negligibly small variations in the intensity of the exposing light results in large variations in the intensity of the light falling on said record.

8. In photographic printing apparatus, a source of light for exposing a sensitive layer through an image bearing record, means for varying the intensity of the light source in accordance with the light falling on said layer including a light sensitive element for varying a current in a circuit as a function of the intensity of the light falling on said layer, a resistance in the circuit for translating the varying current into a varying potential, a source of electrical energy, an amplifier connected between said source of energy and said source of light, a control element subjected to said varying potential for regulating the gain of said amplifier, whereby the intensity of the light source varies through wide limits upon relatively small variations in potential drop across said resistance.

9. In photographic printing apparatus, a source of light for exposing a sensitive layer through an image bearing record, means for ARMANI) G. DENIS. 

